Led driver circuit and led driving method

ABSTRACT

A light emitting diode (LED) driver circuit includes an LED string and a conducting state detection circuit. The conducting state detection circuit detects a conducting state of the LED string, and generates a discharge control signal upon sensing that the LED string is in a non-conducting state. A current source generates a discharge current according to the discharge control signal when the LED string is in the non-conducting state. A passive bleeder provides current compensation by internal regulator operation. An LED spike current suppression circuit suppresses spike current that can occur when the input voltage increases above a threshold. A bias supply circuit has an input capacitor that provides a bias voltage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 15/626,307 filed Jun. 19, 2017 titled “LED DRIVER CIRCUIT AND LEDDRIVING METHOD” (now U.S. Pat. No. ______). U.S. application Ser. No.15/626,307 claimed the benefit of U.S. Prov. App. No. 62/366,688 filedJul. 26, 2016, and also claimed the benefit of U.S. Prov. App. No.62/359,324 filed Jul. 7, 2016. All the noted applications areincorporated by reference herein as if reproduced in in full below.

FIELD OF THE INVENTION

The present invention relates generally to electrical circuits, and moreparticularly but not exclusively to electrical circuits for lightemitting diodes.

BACKGROUND

A light emitting diode (LED) string may be configured with a pluralityof serially-connected LED elements, and may be directly connected to anddriven by an input alternating current (AC) source (“AC input”), such asfrom a wall AC outlet. In that configuration, which is known as DirectAC Drive topology, the AC input may be passed through a dimmer, thenrectified by a rectifier circuit, and then supplied as an AC line to theLED elements. The waveform of the AC input that has passed through thedimmer and the rectifier circuit is referred to herein as the “inputvoltage.” The number of LED elements to be conducted, i.e., turned on,in the LED string can be controlled according to the input voltage.

The AC input provides input current that flows through the dimmer andthe rectifier circuit. The input current should not be lower than aholding current in order to keep the dimmer turned on. When the inputcurrent decreases below the holding current, the dimmer turns off,thereby causing the AC input to be separated from the AC line.Accordingly, when the dimmer is turned off, the input voltagefluctuates, resulting in flicker and difficulty of predicting the inputvoltage.

When the input current is lower than a certain level, a bleedingoperation is initiated in order to regulate the input current. Duringthe bleeding operation, a bleeding circuit causes current to flow fromthe AC line through a metal oxide semiconductor field effect transistor(MOSFET) of the bleeding circuit. The bleeding operation increases powerconsumption and generates heat that causes the temperature of the MOSFETto rise.

SUMMARY

In one embodiment, a light emitting diode (LED) driver circuit includesan LED string and a conducting state detection circuit. The conductingstate detection circuit detects a conducting state of the LED string,and generates a discharge control signal upon sensing that the LEDstring is in a non-conducting state. A current source generates adischarge current according to the discharge control signal when the LEDstring is in the non-conducting state. The LED driver circuit mayinclude a passive bleeder to provide current compensation by internalregulator operation. The LED driver circuit may include an LED spikecurrent suppression circuit to suppress LED spike current that can occurwhen the input voltage increases above a threshold. The LED drivercircuit may include a bias supply circuit that has an input capacitor toprovide a bias voltage.

These and other features of the present invention will be readilyapparent to persons of ordinary skill in the art upon reading theentirety of this disclosure, which includes the accompanying drawingsand claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of an LED driver circuit in accordancewith an embodiment of the present invention.

FIG. 2 shows waveforms of signals of the LED driver circuit of FIG. 1 inaccordance with an embodiment of the present invention.

FIG. 3 shows further details of the LED conducting state detectioncircuit and the current source of FIG. 1, in accordance with anembodiment of the present invention.

FIG. 4 shows waveforms of signals of the LED driver circuit of FIG. 3 inaccordance with an embodiment of the present invention.

FIG. 5 shows a schematic diagram of an LED driver circuit in accordancewith another embodiment of the present invention.

FIG. 6 shows further details of an LED current controller in accordancewith an embodiment of the present invention.

FIG. 7 shows waveforms of signals of the LED driver circuit of FIG. 6 inaccordance with an embodiment of the present invention.

FIG. 8 shows a schematic diagram of an LED driver circuit in accordancewith another embodiment of the present invention.

FIG. 9 shows a schematic diagram of a regulator of an LED currentcontroller in accordance with an embodiment of the present invention.

FIG. 10 shows a schematic diagram of an input threshold voltagegenerator in accordance with an embodiment of the present invention.

FIG. 11 shows waveforms of signals of an LED driver circuit with theregulator of FIG. 9, in accordance with an embodiment of the presentinvention.

FIG. 12 shows a zoom-in view of waveforms of signals in the example ofFIG. 11 in accordance with an embodiment of the present invention.

FIG. 13 shows a schematic diagram of an LED driver circuit in accordancewith another embodiment of the present invention.

FIG. 14 shows a schematic diagram of a bias supply circuit in accordancewith an embodiment of the present invention.

FIG. 15 shows waveforms of signals of the bias supply circuit of FIG. 14in accordance with an embodiment of the present invention.

The use of the same reference label in different drawings indicates thesame or like components.

DETAILED DESCRIPTION

In the present disclosure, numerous specific details are provided, suchas examples of circuits, components, and methods, to provide a thoroughunderstanding of embodiments of the invention. Persons of ordinary skillin the art will recognize, however, that the invention can be practicedwithout one or more of the specific details. In other instances,well-known details are not shown or described to avoid obscuring aspectsof the invention.

FIG. 1 shows a schematic diagram of an LED driver circuit 1 inaccordance with an embodiment of the present invention. In the exampleof FIG. 1, the LED driver circuit 1 includes a rectifier circuit 3, anLED string 2, an LED current controller 4, a current source 5, aconducting state detection circuit 6, a dimmer 7, and a capacitor C1.

The capacitor C1 is connected to an AC line to which the input voltageVin is supplied via the rectifier circuit 3. The capacitor C1 filtersthe ripples of the input voltage Vin.

The dimmer 7 is connected between the AC input ACin and the rectifiercircuit 3. The dimmer 7 may be a phase-cut dimmer that passes only theAC input ACin belonging to a certain range of phases, which is referredto herein as “phase angle”. The AC input ACin passed through the dimmer7 is rectified by the rectifier circuit 3 to generate the input voltageVin. The waveform of the input voltage Vin may have only a portioncorresponding to the phase angle.

The LED string 2 includes a plurality of LED elements LED1-LEDn that areconnected in series with one another. The input voltage Vin is providedto the LED string 2. While FIG. 1 illustrates a plurality of LEDelements, the LED string 2 may be configured with only one LED element.

The controller 4 controls current flowing through a plurality ofchannels CH_1-CH_n. Each of the plurality of channels CH_1-CH_n ispositioned between a corresponding LED element of the plurality of LEDelements LED1-LEDn and a corresponding regulator of a plurality ofregulators 40_1-40_n. In one embodiment, each of the plurality ofregulators 40_1-40_n comprises a linear regulator.

In the example of FIG. 1, one sense resistor RCS is connected to thecontroller 4, and current flowing through each of the plurality ofchannels CH_1-CH_n is controlled according to one sense voltage VCS thatis developed on the sense resistor. Current may flow through one of theplurality of channels CH_1-CH_n. Accordingly, the current ILED flowingto the LED string 2 may flow through any one of the plurality ofchannels CH_1-CH_n to the sense resistor RCS to develop the sensevoltage VCS.

The controller 4 includes the plurality of regulators 40_1-40_nconnected to the plurality of channels CH_1-CH_n, respectively. One ofthe plurality of regulators 40_1-40_n may be enabled according to theinput voltage Vin and control current flowing through a correspondingchannel. In one embodiment, each of the plurality of regulators40_1-40_n may control different amounts of currents.

Each of the plurality of regulators 40_1-40_n is connected to acorresponding channel of the plurality of channels CH_1-CH_n, receives acorresponding control voltage of the plurality of control voltagesVC_1-VC_n, and controls the sense voltage VCS with the control voltageto thereby control the current of the corresponding channel. Each of theplurality of control voltages VC_1-VC_n is the voltage that determinesthe current flowing through a corresponding channel and may be setaccording to the corresponding channel. In one embodiment, the controlvoltages VC_1-VC_n have different values with VC_n being the highest andVC_1 being the lowest. For example, each control voltage VC may be afixed voltage with a relationship VC_1<VC_2< . . . <VC_n. That is, thecontrol voltage VC_1 corresponding to the regulator 40_1 is lower thanthe control voltage VC_2 corresponding to the regulator 40_2, thecontrol voltage VC_2 corresponding to the regulator 40_2 is lower thanthe control voltage VC_3 corresponding to the regulator 40_3, etc.

For convenience of explanation, it is assumed hereinbelow that when thecontrol voltage level corresponding to the LED element LED1 is 1, thecontrol voltage level that corresponds to the LED element LEDn is n.

In the example of FIG. 1, the number of LED elements that will conductis determined according to the input voltage Vin and, among the channelsconnected to the conducting LED elements, a regulator of a channel inthe highest position is enabled. The current of the correspondingchannel is controlled by the enabled regulator based on thecorresponding control voltage. Referring to the LED string illustratedin FIG. 1, the channel is in the higher position as the position of theLED element is closer to the right-hand side.

An example operation of the controller 4 when the input voltage Vinincreases is now explained. As the input voltage Vin increases, thefirst LED element LED1 conducts so that current flows through thechannel CH_1. The regulator 40_1 controls the current ILED so that thelevel of the sense voltage VCS follows the level “1”.

According to the increasing input voltage Vin, the second LED elementLED2 conducts so that the current ILED flows through the channel CH_2.The regulator 40_2 controls the current ILED so that the level of thesense voltage VCS follows the level “2”. At this time, the level of thesense voltage VCS is higher than “1”, and the regulator 40_1 isdisabled.

When the input voltage Vin increases to a certain level, the (n)th LEDelement LEDn conducts so that the current ILED flows through the channelCH_n. The regulator 40_n controls the current ILED so that the level ofthe sense voltage VCS follows the level “n”. At this time, the level ofthe sense voltage VCS is higher than “n−1”, and the regulator 40_n−1 isdisabled.

Next, an example operation of the controller 4 as the input voltage Vindecreases is explained. According to the decreasing input voltage Vin,the (n)th LED element LEDn is turned off so that current does not flowthrough the channel CH_n and the regulator 40_n is disabled.Accordingly, the regulator 40_n−1 is enabled and the current ILED flowsthrough the channel CH_n−1.

According to the decreasing input voltage Vin, the LED elements areturned off in the order of the (n−1)th LED element, the (n−2)th LEDelement, ( . . . ), the second LED element LED2, and the first LEDelement LED1 and, accordingly, the regulators are enabled in the orderof the regulator 40_n−2, the regulator 40_n−3, ( . . . ), the regulator40_2, and the regulator 40_1. The current ILED flows through thecorresponding channels in the order of the channel CH_n−2, the channelCH_n−3, ( . . . ), the channel CH_2, and the channel CH_1.

In the example of FIG. 1, the plurality of regulators 40_1-40_n of thecontroller 4 share the sense resistor RCS, and the plurality of controlvoltages VC_1-VC_n are set differently such that the regulators to beenabled among the plurality of regulators 40_1-40_n are changed inaccordance with a change in the input voltage Vin, and the target valueof the corresponding channel current is also changed.

The operation of the controller 4 has been described above by way ofexample; the present disclosure is not limited thereto.

The input voltage Vin decreases when the dimmer 7 turns off according toa decrease in the input current Iin. Due to the decrease in the inputvoltage Vin, the plurality of LED elements LED1-LEDn transition to anon-conducting state. Hereinafter, the “LED non-conducting state” refersto a state in which all of the plurality of LED elements LED1-LEDn arenot conducting so that the current ILED does not flow to the LED string2. In the LED non-conducting state, current does not flow through any ofthe plurality of channels CH_1-CH_n.

In response to sensing the LED non-conducting state, the LED drivercircuit 1 controls the input voltage Vin by discharging the capacitorC1. The LED driver circuit 1 may sense the LED non-conducting state fromthe sense voltage VCS, from a voltage at an end of one of the pluralityof LED elements LED1-LEDn, or from an output of a regulator amplifier ofthe controller 4.

The LED conducting state detection circuit 6 receives the sense voltageVCS or a voltage at an end of one of the plurality of LED elementsLED1-LEDn, and when the sense voltage VCS or the voltage at an end ofone of the plurality of LED elements LED1-LEDn decreases below adischarge threshold voltage, controls the discharge current IS flowingto the current source 5. The LED conducting state detection circuit 6may also control the discharge current IS flowing to the current source5 when the LED non-conducting state is detected from an output of aregulator amplifier of the controller 4.

Because the current ILED does not flow in the LED non-conducting state,the sense voltage VCS is not generated and may be lower than thedischarge threshold voltage. Also, the input voltage Vin may bedecreased in the LED non-conducting state such that the voltage at anend of one of the plurality of LED elements LED1-LEDn may be lower thanthe discharge threshold voltage.

Upon sensing the LED non-conducting state, the LED conducting statedetection circuit 6 may generate a discharge control signal DCS tocontrol the current flowing through the current source 5.

The current source 5 generates the discharge current IS according to thedischarge control signal DCS, and the capacitor C1 is discharged inaccordance with the discharge current IS. It is thus possible to controlthe input voltage Vin in the LED non-conducting state, without the powerconsumption associated with regulating the input current Iin to theholding current.

FIG. 2 shows waveforms of signals of the LED driver circuit of FIG. 1 inaccordance with an embodiment of the present invention. FIG. 2 shows,from top to bottom, the input voltage Vin, the sense voltage VCS, andpossible waveforms of the discharge current IS.

In the example of FIG. 2, the LED non-conducting state is sensed usingthe sense voltage VCS for illustration purposes only. For example, ananode voltage or a cathode voltage of the LED element LED1 may be usedto sense the LED non-conducting state in other embodiments. In theexample of FIG. 2, the waveforms (a)-(h) illustrate different ways ofgenerating the discharge current IS by way of example.

As illustrated in FIG. 2, according to the decrease in the input voltageVin, the LED non-conducting state occurs at time point T1, and the sensevoltage VCS reaches zero voltage that is lower than the dischargethreshold voltage at the time point T1.

As shown in (a) and (b) of FIG. 2, the LED conducting state detectioncircuit 6 may be synchronized at time point T1 to generate a dischargecontrol signal DCS to increase the discharge current IS. Alternatively,as shown in (c) and (d) of FIG. 2, the LED conducting state detectioncircuit 6 may be synchronized to generate the discharge control signalDCS to increase the discharge current IS at time point T2, which isdelayed from time point T1 by a delay period TD.

As shown in (e), (f), (g) and (h) of FIG. 2, the discharge current ISmay flow with a certain offset level before time point T1. This is onlyto illustrate that the offset level discharge current IS can flow for aduration other than in the LED non-conducting state.

As shown in (e) and (f) of FIG. 2, the LED conducting state detectioncircuit 6 may be synchronized at time point T2 to generate a dischargecontrol signal DCS to increase the discharge current IS.

As shown in (g) and (h) of FIG. 2, the LED conducting state detectioncircuit 6 may block the discharge current IS at time point T1, and besynchronized at time point T2 to generate a discharge control signal DCSto increase the discharge current IS.

The current source 5 may generate the discharge current IS into avariety of waveforms like those illustrated in FIG. 2 (a)-(h) accordingto the discharge control signal DCS. In the LED non-conducting state,the input voltage Vin is decreased by the discharge current IS. In theLED non-conducting state, the waveform of the decreasing input voltageVin may be controlled according to the waveform of the discharge currentIS.

The LED conducting state detection circuit 6 and the current source 5are now described with reference to the LED driver circuit 1 shown inFIG. 3.

FIG. 3 shows further details of the LED conducting state detectioncircuit 6 and the current source 5 in accordance with an embodiment ofthe present invention. Components illustrated in FIG. 3 that overlapwith those illustrated in FIG. 1 are indicated with the same referencenumerals and not redundantly described below. Although FIG. 3 depictsthe LED conducting state detection circuit 6 using a cathode voltage ofthe LED element LED1 as an example, the present disclosure is notlimited thereto. In the example of FIG. 3, the cathode voltage of theLED element LED1 is the input voltage Vin subtracted by a forwardvoltage of the LED element LED1, and thus varies according to the inputvoltage Vin.

In the example of FIG. 3, the LED conducting state detection circuit 6includes three resistors R1-R3, a transistor 61, a capacitor C2, and aZener diode 62. The transistor 61 may be implemented as an NPN bipolarjunction transistor (BJT), although it may be implemented using othertypes of transistors.

The resistor R1 and the resistor R2 are connected in series between thecathode of the LED element LED1 and the ground, and resistive-divide thecathode voltage of the LED element LED1. The resistive-divided voltageis supplied to the base of the transistor 61.

A collector of the transistor 61 is connected to a node N1, and anemitter of the transistor 61 is connected to the ground. The resistor R3is connected between the voltage source VS and the node N1. In theexample of FIG. 3, the voltage source VS is the means for charging thecapacitor C2. In other configurations, the current source 5 may be usedto charge the capacitor C2.

The capacitor C2 and the Zener diode 62 are connected in parallelbetween the node N1 and the ground. The voltage on the capacitor C2controls the voltage of the node N1 as the capacitor C2 is charged ordischarged according to a switching operation of the transistor 61. TheZener diode 62 may clamp the voltage of the node N1 to the Zenervoltage. The discharge control signal DCS follows the voltage of thenode N1.

When the cathode voltage of the LED element LED1 is decreased accordingto a decrease in the input voltage Vin and reaches the dischargethreshold voltage, the base voltage of the transistor 61 is decreasedand thereby turns off the transistor 61. Accordingly, the capacitor C2is charged with the current flowing from the voltage source VS throughthe resistor R3 such that the voltage of the node N1 is increased. Thevoltage of the node N1 may be increased to the Zener voltage. That is,the discharge control signal DCS may be increased starting from a timepoint when the transistor 61 is turned off according to the decrease inthe input voltage Vin, and clamped to the Zener voltage of the Zenerdiode 62.

The transistor 61 turns on when the cathode voltage of the LED elementLED1 is higher than the discharge threshold voltage. In that case,current flows to the ground through the transistor 61, and the capacitorC2 is not charged.

In the example of FIG. 3, the current source 5 includes a transistor 51and a resistor R4. The transistor 51 may be implemented as an n-channeltype MOSFET, although it may be implemented as other types oftransistors.

The discharge control signal DCS is supplied to a gate of the transistor51, and a drain of the transistor 51 is connected to the AC line (seealso FIG. 1). A source of the transistor 51 is connected to one end ofthe resistor R4. The flow of the discharge current IS through thetransistor 51 may be controlled according to the discharge controlsignal DCS.

FIG. 4 shows waveforms of signals of the LED driver circuit 1 of FIG. 3in accordance with an embodiment of the present invention. FIG. 4 shows,from top to bottom, the input voltage Vin, the cathode voltage VLED1 ofthe LED element LED1, the discharge current IS, the discharge controlsignal DCS, and the input current Iin.

The cathode voltage illustrated in FIG. 4 is the cathode voltage of theLED element LED1, which is indicated by VLED1. The input voltage Vinillustrated in FIG. 4 is an input voltage generated by a leading edgedimmer 7. The waveforms are illustrated in FIG. 4 only by way of exampleto describe the exemplary embodiments, and the present disclosure is notlimited thereto.

In the example of FIG. 4, the dimmer 7 is turned on at time point T10 sothat the input voltage Vin is generated and firing of the input currentIin is generated. A number of LED elements according to the level of theinput voltage Vin conduct and the current ILED flows to the LED string2. During the ON period of the dimmer 7, the input current Iin followsthe current ILED and, accordingly, the input current Iin may also changeaccording to the number of LED elements that conduct according to thelevel of the input voltage Vin.

When the input voltage Vin reaches its peak and then decreases due tothe decrease in the input current Iin, the dimmer 7 turns off at timepoint T11. While it is illustrated by way of example that the cathodevoltage VLED1 becomes zero voltage and the input current Iin does notflow at time point T11, this is only an example provided to explain theembodiments, and the present disclosure is not limited hereto.

The cathode voltage VLED1 is lower than the discharge threshold voltageat time point T11, and the LED conducting state detection circuit 6generates the discharge control signal DCS. The current source 5supplies the discharge current IS according to the discharge controlsignal DCS. The discharge current IS may be increased to the indexwaveform according to voltage-current characteristic of the transistor51.

While FIG. 4 illustrates that the discharge current IS starts to flowwithout delay upon sensing LED non-conducting state, a delay may occurbetween time point T11 and the time point of generating the dischargecurrent IS as noted above with reference to FIG. 2. Furthermore, thedischarge current IS may flow with an offset level before time pointT11.

Continuing with the example of FIG. 4, the capacitor C1 is discharged bythe discharge current IS and the falling waveform of the input voltageVin is controlled according to the waveform of the discharge current IS.When the input voltage Vin reaches zero voltage at time point T12, thedischarge current IS may not flow anymore. The discharge control signalDCS may rise and be clamped to the Zener voltage VZ. The dimmer 7 isturned on again at time point T13 and the operation from time point T10to time point T12 repeats.

Rather than regulating the input current Iin to a holding current, whichis the method used in the related art in order to prevent the dimmerfrom turning off, the LED driver circuit 1 according to an exemplaryembodiment controls the input voltage Vin by discharging the capacitorC1 after the dimmer 7 is turned off. By doing so, problems such asvariations in the input voltage Vin, flicker, rising temperature ofMOSFET, and so on can be alleviated.

Inrush current may be generated when the dimmer 7 is turned on. Asillustrated in FIG. 4, the input current Iin has a firing due toinfluence of the inrush current. The LED driver circuit may furtherinclude a passive bleeder in order to prevent excessive rising of theinput current Iin due to the inrush current. However, when the inputvoltage Vin starts to fall with a negative slope after the peak,negative current may flow to the passive bleeder, thus resulting in aproblem of decreasing input current Iin.

FIG. 5 shows a schematic diagram of an LED driver circuit 10 inaccordance with an embodiment of the present invention.

In the example of FIG. 5, the LED driver circuit 10 includes a passivebleeder 11. The components of the LED driver circuit 10 illustrated inFIG. 5 that overlap with those illustrated in FIG. 1 are indicated withthe same reference numerals and symbols.

In the example of FIG. 5, the passive bleeder 11 is connected between anode N2 to which the input voltage Vin is supplied and a node N3 wherethe sense voltage VCS is generated. The passive bleeder 11 includes aresistor RP and a capacitor CP.

The bleeder current IBL flows through the resistor RP and the capacitorCP to the node N3. When the input voltage Vin starts to decrease, thebleeder current IBL may have a negative value, and the phase of thebleeder current IBL may have 90 degree of difference from the phase ofthe input current Iin.

FIG. 6 shows further details of an LED current controller 4 inaccordance with an embodiment of the present invention. The LED drivercircuit of FIG. 6 comprises the passive bleeder 11, the controller 4,and the LED string 2.

In the example of FIG. 6, the plurality of regulators 40_1-40_n maycomprise linear regulators. In the example of FIG. 6, the plurality ofregulators 40_1-40_n each includes a transistor 42 connected to acorresponding channel of the plurality of channels CH_1-CH_n, and anamplifier 41 (“regulator amplifier”) for controlling the transistor 42.A drain of the transistor 42 is connected to the corresponding channel,and a source of the transistor 42 is connected to one end of the senseresistor RCS. An output of the amplifier 41 is connected to a gate ofthe transistor 42, an inverting terminal (−) of the amplifier 41 isconnected to a source of the transistor 42, and a non-inverting terminal(+) of the amplifier 41 is inputted with a corresponding control voltageamong the plurality of control voltages VC_1-VC_n.

The amplifier 41 generates an output based on a difference between acorresponding control voltage inputted to the non-inverting terminal (+)and the sense voltage VCS inputted to the inverting terminal (−), andthe transistor 42 controls the current of the corresponding channelaccording to the output from the amplifier 41. Then the voltages of thenon-inverting terminal (+) and the inverting terminal (−) of theamplifier 41 are regulated to be equal.

As can be appreciated, one way of detecting the LED non-conducting stateis to monitor the output of the amplifier 41. When the output of theamplifier 41 is pulled up, it indicates that the input voltage Vin islower than the forward voltage of the corresponding LED element and theLED current ILED does not flow. Another way of detecting the LEDnon-conducting state is to monitor the sense voltage VCS. When the sensevoltage VCS is lower than a certain level, it indicates that theconduction of the LED string is almost finished. Accordingly, an LEDconducting state detection circuit 6 may be configured to detect the LEDnon-conducting state by monitoring an end (cathode or anode) of an LEDelement, the output of the amplifier 41, and/or the sense voltage VCS.

Even when the bleeder current IBL is a negative value, the input currentIin may be compensated because the voltage of the node N3 is controlledto the sense voltage VCS by the enabled regulator among the plurality ofregulators 40_1-40_n.

When the passive bleeder 11 is connected between the node N2 and theground, the input current Iin decreases as much as a bleeder current IBLwhen the bleeder current IBL is a negative value. However, in anotherexemplary embodiment, even when the bleeder current IBL is a negativevalue, because the voltage of the node N3 is regulated to the sensevoltage VCS, the current ILED flowing through a channel corresponding tothe enabled regulator is increased as much as the bleeder current IBL.

Because the input current Iin is the sum of the bleeder current IBL andthe current ILED, the current ILED may be increased as much as thebleeder current IBL is decreased. Accordingly, the input current Iin canbe compensated.

FIG. 7 shows waveforms of signals of the LED driver circuit of FIG. 6 inaccordance with an embodiment of the present invention. FIG. 7 shows,from top to bottom, the input voltage Vin, the sense voltage VCS, thebleeder current IBL, and the input current Iin.

As illustrated in FIG. 7, the bleeder current IBL is generated as anegative value after time point T20 of a peak of the input current Vin.Advantageously, because the sense voltage VCS is regulated to thecontrol voltage, the input current Iin is not decreased due to thebleeder current IBL. Without the bleeder current IBL being connected tothe node N3, the input current Iin may decrease as indicated by thephantom line in the waveform of the input current Iin.

The embodiment illustrated in FIGS. 1 and 3 may be combined with theembodiment illustrated in FIG. 5 as shown in FIG. 8.

FIG. 8 shows a schematic diagram of an LED driver circuit 100 inaccordance with an embodiment of the present invention. As illustratedin FIG. 8, the LED driver circuit 100 may include the current source 5,the LED conducting state detection circuit 6, and the passive bleeder11. The configuration and operation of the LED driver circuit 100 are aspreviously described.

FIG. 9 shows a schematic diagram of a regulator of an LED currentcontroller in accordance with an embodiment of the present invention.The regulator of FIG. 9 may be employed as a regulator in an LED currentcontroller 4. In the example of FIG. 9, the regulator is illustrated asa regulator 40_1 of the controller 4. It is to be noted that theregulator of FIG. 9 may be employed in other LED current controllers.

A regulator 40_1 may include the regulator amplifier 41 and thetransistor 42 as previously explained with reference to FIG. 6. Thedrain of the transistor 42 is connected to a cathode of the LED elementLED1 of the LED string 2. The inverting (−) input of the amplifier 41and the source of the transistor 42 are connected to the sense voltageVCS that is developed on the sense resistor RCS. The operation of thesecomponents are as previously explained.

In the example of FIG. 9, the regulator 40_1 further includes a spikecurrent suppression circuit comprising a transistor 45. The gate of thetransistor 45 is driven by an input threshold voltage VIN.TH through aninverter.

FIG. 10 shows a schematic diagram of an input threshold voltagegenerator in accordance with an embodiment of the present invention. Inthe example of FIG. 10, a resistive divider comprising resistors R10 andR11 scales the input voltage Vin to generate a voltage that isindicative of the input voltage Vin, which is input to an amplifier 46.The amplifier 46 compares the scaled input voltage Vin to a thresholdvoltage Vth to generate the input threshold voltage VIN.TH. The inputthreshold voltage VIN.TH is high when the scaled input voltage Vin isgreater than the threshold voltage Vth, and the input threshold voltageVIN.TH is low when the scaled input voltage Vin is lower than thethreshold voltage Vth. An optional delay 47 may be added to the outputof the amplifier 46.

Referring back to FIG. 9, when the input threshold voltage VIN.TH islow, the transistor 45 is on and pulls down the output G(1) of theamplifier 41. Because the output G(1) is pulled down, LED spike currentthat may occur when the input voltage VIN increases back up issuppressed.

FIG. 11 shows waveforms of signals of an LED driver circuit with theregulator of FIG. 9, in accordance with an embodiment of the presentinvention. FIG. 11 shows, from top to bottom, the input voltage Vin, theinput threshold voltage VIN.TH, the sense voltage VCS, and the outputG(1) of the amplifier 41. Also shown in FIG. 11 are the cathode voltageVLED1 of the LED element LED1 and a threshold voltage Vth. As can beappreciated, the input voltage Vin and/or the threshold voltage Vth maybe scaled depending on implementation details.

As shown in FIG. 11, the input threshold voltage VIN.TH is high when theinput voltage Vin is higher than the threshold voltage Vth, and is lowwhen the input voltage Vin is lower than the threshold voltage Vth. Theoutput G(1) decreases as the sense voltage VCS increases, and increasesas the sense voltage VCS decreases. The sense voltage VCS drops to zerowhen the input voltage Vin decreases below the cathode voltage VLED1 ofthe LED element LED1, entering the LED non-conducting state. A zoom-inview at time T30 is shown in FIG. 12.

FIG. 12 shows a zoom-in view of waveforms of signals in the example ofFIG. 11 at time T30. FIG. 12 shows, from top to bottom, the inputvoltage Vin, the input threshold voltage VIN.TH, the output G(1) of theamplifier 41, and the sense voltage VCS. FIG. 12 shows the input voltageVin as rectangular for ease of illustration.

As shown in FIG. 12, the input threshold voltage VIN.TH is high when theinput voltage Vin increases above the threshold voltage Vth at time T30.This turns off the transistor 45, thereby removing the pull down andallowing the output G(1) to increase and turn on the transistor 42.Turning on the transistor 42 allows current to flow through the channelCH_1 and to the sense resistor RCS, thereby causing the sense voltageVCS to increase. Because of the pull down on the output G(1) when theinput voltage Vin is below the threshold voltage Vth, a spike currentthat may occur at the time T30 (see dotted area 201) is suppressed.

FIG. 13 shows a schematic diagram of an LED driver circuit in accordancewith an embodiment of the present invention. The LED driver circuit ofFIG. 13 includes the previously described LED string 2, rectifiercircuit 3, LED current controller 4, dimmer 7, and sense resistor RCS.

In the example of FIG. 13, the LED driver circuit further includes abias supply circuit 300 for providing a bias voltage BIAS1 to thecontroller 4 and, optionally, a bias voltage BIAS2 to a peripheraldevice 301. The bias supply circuit 300 may be a low dropout regulator(LDO) or simply a resistor that is connected to the input voltage Vin.However, that implementation would lead to large power losses as theinput voltage Vin becomes higher.

FIG. 14 shows a schematic diagram of a bias supply circuit 300 inaccordance with an embodiment of the present invention. In the exampleof FIG. 14, the bias supply circuit 300 includes a capacitor C21, adiode D1, and a diode D2. One end of the capacitor C21 is connected tothe input voltage Vin, and the other end of the capacitor C21 isconnected to an anode of the diode D1 and to a cathode of the diode D2.The bias supply circuit 300 may optionally include a resistor R20 thatis in series with the capacitor C21 to form a passive bleeder. The diodeD2 may optionally be a Zener diode (see 310) to limit the bias voltageif the bias voltage is too high due to excessive energy from thecapacitor C21. The phase-cut dimmer 7 (shown in FIG. 13) is optional andcan be added to modulate the input voltage Vin.

In the example of FIG. 14, the anode of the diode D2 is connected toground, and the cathode of the diode D2 is connected to the anode of thediode D1 and to the other end of the capacitor C21. The anode of thediode D1 is connected to the cathode of the diode D2 and to the otherend of the capacitor C21. The cathode of the diode D1 provides the biasvoltage BIAS to the controller 4, peripheral device 301, and/or othercircuits.

In the example of FIG. 14 a capacitor current I_C21 flows to thecapacitor C21 from the input voltage Vin, a first diode current I_D1flows from the anode to the cathode of the diode D1, and a second diodecurrent I_D2 flows from the cathode to the anode of the diode D2. FIG.15 shows the waveforms and timing relationships of these signals in oneembodiment. FIG. 15 shows, from top to bottom, the input voltage Vin,the capacitor current I_C21, the first diode current I_D1, and thesecond diode current I_D2.

LED driver circuits and methods of operating same have been disclosed.While specific embodiments of the present invention have been provided,it is to be understood that these embodiments are for illustrationpurposes and not limiting. Many additional embodiments will be apparentto persons of ordinary skill in the art reading this disclosure.

What is claimed is:
 1. A light emitting diode (LED) driver circuit,comprising: an LED string having a first end that is connected to aninput voltage, the LED string comprising at least one LED element; atleast one channel connected to the at least one LED element; a senseresistor having a first end that is connected to the at least onechannel and a second end that is connected to ground; at least oneregulator connected between the at least one channel and the first endof the sense resistor and configured to control current flowing throughthe at least one channel based on a control voltage; and a passivebleeder connected between the input voltage and the first end of thesense resistor.
 2. The LED driver circuit of claim 1, wherein the atleast one regulator comprises: a transistor connected between the atleast one channel and the first end of the sense resistor; and anamplifier comprising a first input connected to the control voltage, asecond input connected to the first end of the sense resistor, and anoutput connected to a gate of the transistor.
 3. The LED driver circuitof claim 1, wherein the passive bleeder comprises a resistor and acapacitor connected in series between the input voltage and the firstend of the sense resistor.
 4. A light emitting diode (LED) drivercircuit, comprising: an LED string having a first end that is connectedto an input voltage, the LED string comprising at least one LED element;at least one channel connected to the at least one LED element; a senseresistor having a first end that is connected to the at least onechannel; at least one regulator connected between the at least onechannel and the first end of the sense resistor and configured tocontrol current flowing through the at least one channel based on acontrol voltage; and a spike current suppression circuit that isconfigured to pull down an output of an amplifier of the at least oneregulator when the input voltage is below a threshold voltage.
 5. TheLED driver circuit of claim 4, wherein the at least one regulatorcomprises: a transistor connected between the at least one channel andthe first end of the sense resistor; and the amplifier comprising afirst input connected to the control voltage, a second input connectedto the first end of the sense resistor, and the output connected to agate of the transistor.
 6. The LED driver circuit of claim 5, whereinthe spike current suppression circuit comprises: another transistorhaving a drain connected to the output of the amplifier, a source thatis connected to ground, and a gate that is connected to receive an inputthreshold voltage that is indicative of a level of the input voltage. 7.A light emitting diode (LED) driver circuit, comprising: an LED stringhaving a first end that is connected to an input voltage, the LED stringcomprising at least one LED element; at least one channel connected tothe at least one LED element; an LED current controller that isconfigured to regulate current through the at least one channel; and abias supply circuit that is configured to provide a bias voltage to theLED current controller, the bias supply circuit comprising a capacitor,a first diode, and a second diode, wherein a first end of the capacitoris connected to the input voltage, a second end of the capacitor isconnected to an anode of the first diode, a cathode of the second diodeis connected to the second end of the capacitor, and a cathode of thefirst diode provides the bias voltage.
 8. The LED driver circuit ofclaim 7, wherein the anode of the second diode is connected to ground.9. The LED driver circuit of claim 7, wherein the second diode is aZener diode.
 10. The LED driver circuit of claim 7, further comprising aresistor that is in series with the capacitor.
 11. The LED drivercircuit of claim 7, further comprising a dimmer that is configured tomodulate the input voltage.
 12. The LED driver circuit of claim 7,wherein the LED current controller comprises at least one regulator, theat least one regulator comprising: a transistor connected between the atleast one channel and a sense resistor; and an amplifier comprising afirst input connected to a control voltage, a second input connected tothe sense resistor, and an output connected to a gate of the transistor.